Multiple ratio mechanical transmissions, whether manual, automatic, or semi-automatic, utilize a fluid disposed within the transmission to accomplish various functions. For example, manual mechanical transmissions use a fluid for lubrication of rotating and meshing parts. Automatic transmissions use a fluid for generating and transferring hydrodynamic power in addition to providing lubrication for rotating parts. The specific formulation and properties of a transmission fluid are generally adapted to the particular application. However, regardless of the application, the various types of transmission fluids have some common properties.
Since vehicles, such as tractor semi-trailer trucks, are often subjected to extreme temperature variations, it is desirable for a transmission fluid to function somewhat consistently over the vehicle operating range. In addition to variations in ambient operating temperatures, a transmission fluid is also subjected to heat generated by friction produced by the numerous rotating components. This temperature fluctuation is manifested as a change in viscosity of the transmission fluid. The change in viscosity may pose a number of challenges in designing a mechanical transmission for consistent performance over a typical operating range.
Ambient and operating temperature variations also result in expansion and contraction of transmission fluid, which results in fluctuations in the transmission fluid volume. This results in a pressure differential between the interior of the transmission and the exterior which is subjected to atmospheric pressure. Such pressure differences could cause failure of the transmission seals. Thus, it is desirable to control the pressure differential between the interior and exterior of a transmission.
A pressure differential may also be created by changes in atmospheric pressure as a vehicle travels from a lower elevation to a higher elevation. Although this change may seem insignificant, atmospheric pressure can vary significantly from elevations ranging from below sea level to those exceeding 10,000 feet.
Thus, a number of transmission manufacturers have added a vent or breather to their transmissions to equalize interior and exterior pressures. For many applications, a small vent tube which is strategically positioned provides this function. However, many of the prior art devices allow bridging of the transmission fluid which results in unnecessary fluid expulsion. Bridging occurs when the transmission fluid spans the opening of the vent or breather due to the cohesion tension of a viscous fluid. As the transmission warms up, the pressure increases within the transmission creating a bubble within the tube or breather which forces the fluid to be expelled unnecessarily from the transmission.
Due to the nature of the breather's function, breathers are usually positioned on a top, or upwardly facing, surface of the transmission. This is a relatively precarious position which leaves the breathers vulnerable to impacts, especially downwardly directed impacts, which could sufficiently impair the structural integrity of the breather rendering the breathers less effective or inoperable for their intended purposes. Such exemplary impacts could occur by a mechanic or driver inadvertently stepping on the top of the breather, or accidently striking the breather with a tool, while working on a component under the hood or cab of a tractor semi-trailer truck.
Prior art breathers typically have at least one opening to help equalize interior and exterior pressures. This opening is essentially unshielded and can enable external contaminants such as water, dirt and other debris to easily enter the transmission through the breather. A typical prior art breather has a shaft having a hexagonal nut-like member fixed at about the midpoint lengthwise of the shaft. Installation of these types of breathers typically requires the use of a non-top drivable tool, causeing the installation of the breather to be relatively time consuming and difficult.
Thus, it is desirable to provide a transmission breather which provides pressure equalization while reducing or eliminating unnecessary transmission fluid expulsion from the breather during normal operation of the transmission without allowing easy entry of external contaminants into the transmission. It is also desirable to provide a breather which reduces or eliminates fluid escape in the form of vapor. It is also desirable to provide a breather which comprises only a few parts, which can be easily manufactured and which can be easily installed in the transmission with a top-drivable tool, such as a socket wrench or a screwdriver. It is also desirable to provide a breather which is designed to withstand the types of impacts which could occur over the lifetime of the breather and which is corrosion resistant.